1. Technical Field
The present invention relates to an electrophotographic photoreceptor (hereinafter also simply referred to as a “photoreceptor”) used for electrophotographic printers, copying machines, facsimiles and the like, a method for manufacturing the same and an electrophotographic device, and more specifically, relates to an electrophotographic photoreceptor having excellent stain resistance due to an improvement of an additive and a method for manufacturing the same and an electrophotographic device.
2. Background of the Related Art
Electrophotographic photoreceptors are generally required to have the function of holding a surface charge in a dark place, the function of generating charge by receiving light and the function of transporting charge by similarly receiving light. Such photoreceptors include so-called single layer photoreceptors having a single-layer photosensitive layer having all the functions in one layer, and so-called stacked photoreceptors having a photosensitive layer including a stack of functionally discrete layers: a layer which primarily serves to generate charge; and a layer which serves to hold a surface charge in a dark place and transport the charge during photoreception.
For example, Carlson process is applied for electrophotographic image formation using such electrophotographic photoreceptors. Images are formed according to the process by charging a photoreceptor in a dark place; forming an electrostatic image of an original text or drawing onto the surface of the charged photoreceptor; developing the formed electrostatic image with a toner; and transferring and fixing the developed toner image onto a support such as paper. The photoreceptor after transfer of the toner image is subjected to removal of remaining toner and neutralisation and then re-used.
The materials for the electrophotographic photoreceptors used include inorganic photoconductive materials such as selenium, selenium alloys, zinc oxide and cadmium sulphide dispersed in a resin binder; organic photoconductive materials such as poly-N-vinylcarbazole, 9,10-anthracenediol-polyester, pyrazoline, hydrazone, stilbene, butadiene, benzidine, phthalocyanine or bisazo compounds dispersed in a resin binder; and materials obtained by vacuum deposition or sublimation of the foregoing.
Because of, for example, an increase in printing pages due to networking in offices and rapid development in light-weight electrophotographic printing machines, electrophotographic printing machines are required to have further improved durability and sensitivity as well as fast response. It is also strongly required to have low effect due to gases generated in the devices such as ozone and NOx and low variation of image characteristics due to varied operating environment (temperature and humidity of the room).
However, conventional photoreceptors do not necessarily and satisfactorily fulfil the required demand characteristics and have issues as indicated below.
For example, concerning stain resistance, streaks are generated in halftone images because the surface of the photoreceptor is stained with components exuded from the constituent of rollers such as a charging roller and a transfer roller which are always in contact with the photoreceptor.
Concerning stain resistance, Patent Literature 1, hereinafter “PTL1” (see the listing in the following), proposes a method in which a resistive layer of a charging roller is formed with a resin containing an ethylene-butylene copolymer and Patent Literature 2, hereinafter “PTL2”, proposes a method in which a transfer roller is formed with a rubber composition containing an epichlorohydrin rubber as a main rubber component and also containing a filler. However, the methods have not been able to satisfactorily meet the demand in stain resistance.
Concerning variation of characteristics of photoreceptors due to operating environment, a first problem is deterioration of the image characteristics in a low temperature and low humidity environment. Namely, in a low temperature and low humidity environment in general, sensitivity characteristics of photoreceptors are apparently decreased, which reveals degradation of image quality such as a decrease of density of images and deterioration of gradation of halftone images. Image memory may become significant accompanying the degradation of sensitivity characteristics. In a printing process, the image recorded as a latent image in the first revolution of the drum is affected by variation of the potential in the second and later revolution of the drum. As a result, printing may occur in unnecessary places particularly in the case of printing of halftone images. This is the degradation of image quality by the image memories. Particularly in a low temperature and low humidity environment, negative memories are often observed in which light and shade of the printing images are reversed.
Image characteristics may also degrade in a high temperature and high humidity environment. Namely, in a high temperature and high humidity environment in general, mobility of charges in the photosensitive layer is larger than in a normal temperature and normal humidity environment, which may cause image defects including excessive increase of printed density and minute black spots in a wholly white image printing (fogging). The excessive increase of printed density increases toner consumption and destructs minute gradation due to enlarged one dot diameter. In contrast to those seen in a low temperature and low humidity environment, positive memories are often observed in which light and shade of printing images is directly reflected.
The performance degradation due to the temperature and humidity is often caused by absorption and release of moisture in the resin binder in the surface layer or in the charge generation material in the photosensitive layer. To address this problem, a variety of materials have been studies including Patent Literature 3 and 4, hereinafter “PTL 3” and “PTL 4”, which disclose addition of a specific compound to a charge generation layer and Patent Literature 5, hereinafter “PTL 5”, in which a specific polycarbonate polymer charge transport material is used for a surface layer. However, materials have not been found that can satisfactorily achieve various characteristics including suppressing an effect of temperature and humidity to photoreceptors.
Moreover, although the techniques disclosed in Patent Literature 6 and 7 and 8, hereinafter “PTL 6”, “PTL 7” and “PTL 8” could address the problem of degradation of characteristics due to the temperature and humidity conditions, the techniques were not satisfactory with regard to the stain resistance on the surface of photoreceptors.
Further, Patent Literature 9, hereinafter “PTL 9”, proposes an outermost surface layer of a photosensitive layer which contains a certain phthalic ester compound and a certain three-dimensional cross-linked polymer. However, PTL 9 does not refer to the stain resistance of the surface of the photoreceptor or the effect of temperature and humidity. Further, Patent Literature 10, hereinafter “PTL 10”, discloses a phthalic acid compound useful as a pest repellent and Patent Literature 11, hereinafter “PTL 11”, discloses a thermosensitive recording paper having a thermosensitive colour developing layer which contains a certain aromatic compound having four ester groups. However, PTL 10 and PTL 11 do not refer to use of the compounds in photoreceptors.